Foams made from alkenyl aromatic polymers such as polystyrene typically exhibit changes in dimension as the temperature is raised significantly above room temperature. Consideration of the heat distortion temperature of alkenyl aromatic foam is very important when it is being used in a relatively high temperature application close to the service temperature limit of the foam (about 165.degree. F. for a conventional polystyrene foam). If the heat distortion temperature of a foam is too low, it may be subject to disfigurement and/or rupture. The stresses set up during foam manufacture are dissipated as the temperature is raised and the linear dimensions of the foams increase or decrease (depending on foam orientation and whether the cell gas pressure is above or below ambient pressure). The temperature at which significant expansion or shrinkage occurs depends on the glass transition temperature of the polymer matrix, which can be depressed due to plasticization by residual blowing agent and other additives which are soluble in the polymer. These effects can also limit the upper service temperature of the foam. One measure of the upper service temperature, and a test to determine the dimensional stability of the foam as a function of temperature, is the Heat Distortion Temperature Test (ASTM D2126-94) which measures the linear change in three dimensions of a foam when exposed to different temperatures.
A common high temperature application for alkenyl aromatic foams is in roofing. In roofing, the foam is typically employed below a roofing membrane, which is dark and rubber-like, and may reach service temperature limits when underneath a membrane exposed to direct sun-light in the summer months. If the foam becomes distorted, the membrane and the foam may separate to form void pockets, which leaves the membrane with less mechanical support on its under surface. The lack of under-surface support renders the membrane more subject to rupture, which results in water leaking in the roof.
U.S. Pat. Nos. 5,411,687; 5,434,195; 5,557,896; 5,693,687; 5,784,845; and 5,824,710, (the entire contents of which are herein incorporated by reference), disclose open cell foams (i.e. containing 30% or more open cells) as a means of obtaining high heat distortion temperatures. However the high open cell content of these foams can result in inferior thermal insulation performance (due to rapid loss of insulating blowing agent) as well as increased water absorption, both of which are undesirable.
Thus it would be desirable to have a closed cell alkenyl aromatic foam with increased heat distortion temperature and improved dimensional stability which also exhibits good vapor resistance, water resistance, and mechanical strength. Uses for such a foam would include insulation in building and construction, as well as in the preparation of foam film labels for bottles and other containers, where the improved dimensional stability of such labels would minimize any shrinkage or warpage of the label when the labeled bottle cools after fabrication.
We have surprisingly found that foams made from blends of alkenyl aromatic polymers and specific types and amounts of substantially random interpolymers, exhibit increased heat distortion temperatures relative to analogous alkenyl aromatic polymer foams made without substantially random interpolymers even when the foams are predominantly closed cell (i.e., open cell content of 20 volume % or less). Furthermore, compared with corresponding foams made without the interpolymers, the foams of the present invention exhibit similar or better performance in creep tests (such as DIN 18164 and ASTM 3575 suffix BB) and environmental dimensional change (ASTM C578-83) tests, as well as improved tensile strength/elongation (ASTM D614-91) and tear strength/elongation (ASTM D412-87).